Power take-off engagement control system

ABSTRACT

An apparatus and method is disclosed for controlling engagement of a power take-off shaft (PTO) on a vehicle. The apparatus includes a clutch including an input shaft coupled to a power source and an output shaft coupled to the PTO. The apparatus also includes an input speed sensor for sensing a quantity related to the angular velocity of the input shaft, an output shaft speed sensor for sensing angular velocity of the output shaft, and a clutch controller for adjusting pressure of hydraulic fluid applied to the clutch in response to control signals. Torque transmitted between the input and output shafts is responsive to the control signals. The apparatus also includes a control circuit coupled to the speed sensors and the clutch controller. The control circuit is configured to receive signals from the speed sensors and to provide the control signals in response thereto. During engagement of the PTO, the control circuit continuously determines an actual acceleration of the PTO based upon the output shaft velocity, and generates the control signals using a desired acceleration and the actual acceleration. The control circuit may continuously determine the desired acceleration based upon the input speed signal. Alternatively, the control circuit may provide control signals to cause the torque transmitted through the clutch to increase at a rate depending upon the difference between the desired and actual accelerations when the actual acceleration is more than a threshold proportion of the desired acceleration, and to cause the torque to increase at a rate independent of the difference when the actual acceleration is less than the threshold proportion.

FIELD OF THE INVENTION

The present invention relates to a control system for engagement of apower take off (PTO) for an agricultural vehicle such as a tractor. Inparticular, the present invention relates to a control system forcontrolling the engagement of a PTO in which the control systemcontinuously determines (i.e., recalculates) a desired acceleration rateof the PTO. Also, the present invention relates to a control system forcontrolling the engagement of a PTO which provides a clutch withcommands to transmit different amounts of torque depending upon thedifference between an actual acceleration rate of the PTO and thedesired acceleration rate of the PTO, and which provides a command totransmit a low amount of torque when the difference between the actualand desired acceleration rates is large enough to indicate that theclutch is not yet engaged.

BACKGROUND OF THE INVENTION

PTO shafts (or simply “PTOs”) are used on agricultural vehicles such astractors to provide power for equipment or implements such as combines,mowers and spreaders. As the use of PTOs developed, most tractormanufacturers standardized on 1000 RPM and 540 RPM PTOs. Thisstandardization involved the use of a common size shaft and splinearrangement for each RPM rating. When the shaft sizes were standardizedyears ago, tractors had relatively low horsepower (e.g., 30 to 50horsepower). Accordingly, the torque output of a PTO was limited by thehorsepower of the tractor. Modern tractors commonly have horsepowerratings in excess of 100 horsepower. However, the shaft sizes for PTOshave not changed due to the need to maintain compatibility with olderequipment and maintain the standardization for PTOs. Thus, the torqueoutput of PTOs for many modern tractors is no longer limited by thetractor horsepower. Rather, the torque output is limited by the strengthof the PTO and the failure thereof. For very high horsepower tractors(e.g., over 130 horsepower), manufacturers have eliminated the 540 RPMPTO. Due to the gear reduction required to achieve a PTO speed of 540RPM at engine idle, the very high horsepower tractors can apply a torqueto the 540 RPM PTO in excess of that required for the PTO toil. Inaddition to causing PTO failures, the torque produced by the higherhorsepower tractors also can accelerate equipment attached to therespective PTO at a rate which can damage the equipment.

Excessive acceleration of (or application of torques to) a PTO is ofparticular concern during the process of engagement of the PTO from astandstill or zero angular velocity state to a “lock-up” state, at whichthe PTO has an angular velocity equaling that of the engine (or,assuming various gear reductions, etc., an angular velocity that is anappropriate fraction or multiple of the angular velocity of the engine).Relevant components associated with this process of engagement of a PTOare shown in FIGS. 1 and 2 (prior art). FIG. 1 shows, in simplifiedform, a conventional (exemplary) arrangement for transmitting power froman engine 2 (of an agricultural vehicle) to a PTO 1. As shown, PTO 1 iscapable of receiving power from engine 2 by way of a PTO clutch 3. PTOclutch 3 is capable of transmitting power from an input shaft 4, whichreceives power from engine 2, to an output shaft 10, which is in turntypically coupled to PTO 1 by way of one or more gears (not shown). Theamount of power transmitted from engine 2 to PTO 1 depends upon whetherPTO clutch 3 is engaged (i.e., whether plates within the clutch havebeen compressed sufficiently to allow the clutch to transmit torque)and, once the clutch has been engaged, upon the degree of hydraulicfluid pressure applied to the clutch, which determines the amount oftorque that the clutch may transmit from input shaft 4 to the PTO viaoutput shaft 10.

PTO 1 may be coupled, by way of a coupler 15, to an implement inputshaft 5 (supported by an implement attached to the agriculturalvehicle). Typically, implement input shaft 5, which is for receivingpower from PTO 1, is in turn coupled to an implement output shaft 13 fortransmitting the power to attached equipment supported by the implement.In certain embodiments, implement input shaft 5 may be coupled toimplement output shaft 13 by way of an over-running clutch 6.Over-running clutch 6 allows implement input shaft 5 (and PTO 1) totransmit power to implement output shaft 13 but also allows the outputshaft to continue to rotate freely when the input shaft no longer isrotating. As shown in FIG. 2, an exemplary over-running clutch 6includes an arrangement in which an output 7 attached to output shaft 13concentrically surrounds an input 8 attached to input shaft 5. Input 8transmits power to output 7 only when spring-actuated locking pins 9 arefully extended into two locking grooves or notches 11 and when input 8receives power (from PTO 1) causing the input to rotate in acounter-clockwise direction relative to output 7, in which case theinput is coupled to the output. In other circumstances, such as whenlocking pins 9 are not fully extended into locking notches 11 (asshown), or when output 7 rotates in a counter-clockwise directionrelative to input 8 (e.g., when no power is being transmitted fromengine 2 but when output shaft 13 nonetheless is rotating in acounter-clockwise direction), output 7 freely rotates with respect toinput 8 and effectively no power is transmitted between the twoelements.

As shown in prior art FIG. 3A, PTO 1 experiences a rapid change inangular velocity during the PTO engagement process once PTO clutch 3 hasbeen engaged such that power is transmitted from engine 2 to the PTO(e.g., after a time t₁). In order to control PTO acceleration duringthis process of engagement of PTO 1, Case Corporation has developed aPTO clutch control system that monitors the angular velocities of inputshaft 4 and output shaft 10 and controls the acceleration of PTO 1 basedupon these measured velocities, as described in U.S. Pat. No. 5,494,142to Kale and incorporated by reference herein. Based upon the monitoredspeeds of input shaft 4 and output shaft 10, the clutch control systemcalculates a desired acceleration for PTO 1 and also repeatedlycalculates an actual acceleration of the PTO. The desired accelerationis calculated as the ratio of the angular velocity of input shaft 4 (ora quantity directly related to the engine speed of the agriculturalvehicle) to a predetermined amount of time (shown in FIG. 3A as the timeinterval between a time t₃ and a time t₁), and is only calculated once.That is, only one calculated value of the desired acceleration isutilized by the clutch control system throughout the PTO engagementprocess. The actual acceleration is calculated as the ratio of thechange in angular velocity of output shaft 10 (or a quantity related tothe speed of PTO 1) during a particular time interval (the time betweentwo velocity measurements) divided by the time interval. Thepredetermined amount of time with respect to the desired acceleration(the time interval between times t₃ and t₁ ) is chosen to restrict thedesired acceleration to a low enough level so that, if PTO 1 actuallyaccelerated at that rate, no damage to the PTO or to attached equipmentwould occur. For example, the predetermined amount of time may be 2seconds. Depending upon whether the desired acceleration exceeds or isless than the actual acceleration at a given time, the clutch controlsystem causes PTO clutch 3 to transmit, respectively, more or the sametorque such that the actual acceleration approaches the desiredacceleration.

FIG. 3A shows the time variation during the PTO engagement process ofthe actual and desired speeds of PTO 1 and the actual speed of engine 2(or a fraction or multiple thereof, to account for gear reductions oraugmentations occurring between engine 2 and PTO 1), and therebyillustrates a typical PTO acceleration (engagement) pattern using theabove-referenced PTO clutch control system. From an initial time t₀until a time t₁, PTO clutch 3 is not yet engaged and is providing notorque and so the actual speed of PTO 1 remains zero. After time t₁, PTOclutch 3 is engaged; that is, the plates of the clutch have beensufficiently compressed so that the torque transmitted by the clutch iseffectively proportional to the hydraulic fluid pressure applied to theclutch. Consequently, PTO 1 begins to receive torque through PTO clutch3 from engine 2 in proportion to hydraulic fluid pressure controlled bythe PTO clutch control system and begins to accelerate, as shown by theactual PTO speed curve. FIG. 3A also shows a desired PTO speed curvethat has a constant slope reflecting the constant desired accelerationthat would be necessary to cause PTO 1 to attain the measured initialspeed of engine 2 (or the appropriate fraction or multiple thereof)within the predetermined amount of time (the time interval between timest₃ and t₁). As shown, the actual PTO speed typically lags the speed thatwould have occurred if PTO 1 consistently accelerated at the desiredacceleration. However, though the actual acceleration of PTO 1 typicallylags behind the desired acceleration of the PTO, it may exceed thedesired acceleration as well. Throughout the process, the PTO clutchcontrol system recalculates the actual acceleration of PTO 1 and adjuststhe torque transmitted by PTO clutch 3 so that the actual accelerationapproaches the desired acceleration.

Although the PTO clutch control system does allow for a controlledacceleration of PTO 1, the system inaccurately presumes that the speedof engine 2 remains constant during the PTO engagement process andtherefore that a single measured angular velocity of input shaft 4 is anaccurate basis upon which to calculate the desired acceleration. Asshown in FIG. 3A, in practice this assumption is incorrect since thespeed of engine 2 (and input shaft 4) typically droops as torque istransmitted by PTO clutch 3 from input shaft 4 to output shaft 10 (andthen to PTO 1). That is, the speed to which output shaft 10 mustaccelerate in order to reach the speed of input shaft 4 is reduced.Assuming that the actual acceleration of PTO 1 is within a reasonableproximity of the desired acceleration, the time required for the PTO toreach the engine speed (or appropriate fraction or multiple thereof)therefore is reduced from the predetermined time used to calculate thedesired acceleration (the time interval between times t₃ and t₁) to ashortened time, the time interval between a time t₂ and time t₁.Consequently, the acceleration and associated stress experienced by PTO1 is larger than is necessary to accelerate the PTO to the engine speed(or appropriate fraction or multiple thereof) by predetermined time t₃.It should be noted that the actual acceleration of PTO 1 is directlyrelated to the magnitude of droop in engine speed and so, as oneincreases the desired and actual accelerations, one produces greaterengine speed droop and exacerbates the above-described problems.

The above-described PTO clutch control system distinguishes between andresponds to only two operational conditions (i.e., whether the actualacceleration exceeds or is less than a given desired acceleration).However, the control system may be modified to include a proportionaladjustment algorithm wherein the degree of adjustment of the torquetransmitted by PTO clutch 3 depends upon the degree by which the desiredacceleration exceeds the actual acceleration. For example, theproportional adjustment algorithm may distinguish among three differentlevels of difference by which the desired acceleration may exceed theactual acceleration: if the actual acceleration of PTO 1 is less thanthe desired acceleration but greater than two-thirds of the desiredacceleration, the control system increases the torque transmitted by PTOclutch 3 at a slow rate; if the actual acceleration is less thantwo-thirds of the desired acceleration but greater than one-third of thedesired acceleration, the control system increases the torquetransmitted at a medium rate; and if the actual acceleration is lessthan one-third of the desired acceleration, the control system increasesthe torque transmitted at a fast rate. Thus, the proportional adjustmentalgorithm causes the actual acceleration to approach the desiredacceleration at a faster rate as the difference by which the desiredacceleration exceeds the actual acceleration increases. As with theunmodified PTO clutch control system without the proportional adjustmentalgorithm, the modified PTO clutch control system may be configured tomaintain the torque transmitted by PTO clutch 3 at a constant level ifthe actual acceleration exceeds the desired acceleration.

Although use of this modified PTO clutch control system provides for amore nuanced response to differences between the desired and actualaccelerations, such use may also cause a second undesirable stress onPTO 1 (or attached equipment) when operating in conjunction withover-running clutch 6 as a result of non-ideal operation of PTO clutch3, as described below. As described above, over-running clutch 6 onlytransmits power from input 8 to output 7 when locking pins 9 are engagedwith locking notches 11. It is frequently the case that, beforeengagement of PTO 1, locking pins 9 are not engaged with locking notches11 and, instead, input 8 is oriented relative to output 7 such that, forthe locking pins to engage the locking notches, the input must rotatecounter-clockwise a portion of a revolution with respect to the output.This is particularly the case since implement output shaft 13 may haverotated due to spurious movement of the attached equipment while PTOclutch 3 was disengaged (e.g., certain types of attached equipment mayrotate due to contact with the ground as the agricultural vehicle andimplement move forward, even though no power is being transmitted fromengine 2 to the equipment via PTO 1). Also, locking pins 9 may disengagefrom locking notches 11 even when engine 2 is delivering power throughPTO clutch 3 in circumstances where implement output shaft 13 (and theattached equipment) begins rotating at a rate that is faster than therotational rate of PTO 1 (e.g., where engine speed is reduced). Ineither of these situations, it is possible for PTO 1 to receive powerfrom engine 2 via PTO clutch 3 and to rotate (such that input 8 rotatescounter-clockwise with respect to output 7) for a short period of timewithout transmitting any power to implement output shaft 13. This isbecause PTO 1 (and implement input shaft 5) must first rotate a portionof a revolution so that locking pins 9 engage locking notches 11 beforeimplement input shaft 5 engages implement output shaft 13 throughover-running clutch 6. In other words, there may be a “locking delay” inthe response of implement output shaft 13 to power transmitted fromengine 2 due to over-running clutch 6.

While the locking delay is of little concern at those times when PTOclutch 3 is engaged, the locking delay is problematic during engagementof PTO 1 as controlled by the modified PTO clutch control system. Asdescribed above, PTO clutch 3 is modeled as ideally having two distinctoperational states, (a) a first, disengaged state in which the plates ofthe clutch are not compressed and so the clutch does not transmit torquebetween input shaft 4 and output shaft 10 (and then to PTO 1), and (b) asecond, engaged state in which the plates of the clutch are compressedand the clutch transmits torque in an amount approximately directlyrelated to the hydraulic fluid pressure applied to the clutch. However,in practice, PTO clutch 3 may still transmit a small but not negligibleamount of torque from input shaft 4 to output shaft 10 even during thefirst, disengaged state, particularly if the hydraulic fluid pressurewithin the clutch is being increased to compress the plates and to causethe clutch to enter the engaged state. Even though this small amount oftorque is typically insufficient to rotate implement output shaft 13 ifthe shaft is attached to equipment, the torque may be sufficient torotate PTO 1 while locking pins 9 of over-running clutch 6 aredisengaged from locking notches 11 and until such time as output 7 locksto input 8 (that is, the torque may be sufficient to rotate the PTOduring the locking delay). To summarize, PTO clutch 3 may transmitenough torque from input shaft 4 to PTO 1 during the PTO engagementprocess, before the clutch is engaged, that the PTO will rotate from aposition in which locking pins 9 of over-running clutch 6 are disengagedfrom locking notches 11 to the position in which the locking pins areengaged with the locking notches.

This rotation of PTO 1 before engagement of PTO clutch 3 results inundesirable consequences given the design of the modified PTO clutchcontrol system, as shown in FIG. 3B (prior art). As described in U.S.Pat. No. 5,494,142, the PTO engagement process effectively begins when aPTO on/off switch (not shown) is closed by the operator of theagricultural vehicle. After that time, the PTO clutch control system(whether modified or unmodified) determines whether PTO 1 (i.e., outputshaft 10) is rotating as an indication of whether PTO clutch 3 isengaged. Once PTO 1 is determined to be rotating, the PTO clutch controlsystem (whether modified or unmodified) then begins to compare desiredand actual accelerations, and begins to adjust the hydraulic fluidpressure applied to PTO clutch 3 in response to differences between thedesired and actual accelerations in order to accelerate the PTO. Whilethe design of the PTO clutch control system presumes that PTO rotationis a good indication of engagement of PTO clutch 3, as discussed abovePTO 1 may begin to rotate before the clutch is engaged so long as theclutch transmits some torque and locking pins 9 of over-running clutch 6are not engaged with locking notches 11. As shown in FIG. 3B, PTO 1 mayhave a nonzero speed between times t₁ and t_(s), at which time PTOclutch 3 is engaged and controlled acceleration of PTO 1 begins,continuing until some later time, t_(L). Nevertheless, under theseconditions the PTO clutch control system still senses the PTO rotation(between times t₁ and t_(s)) and consequently begins to compare thedesired and actual accelerations and to adjust the hydraulic fluidpressure applied to PTO clutch 3. Because the actual torque transmittedto PTO 1 prior to the engagement of PTO clutch 3 is small (and, in anycase, the PTO stops rotating upon the engagement of locking pins 9 andlocking notches 11 of over-running clutch 6), the difference between thedesired and actual accelerations immediately becomes large. The modifiedPTO clutch control system responds to this large differential (i.e., adifferential in which the actual acceleration is less than one third ofthe desired acceleration) by increasing the hydraulic fluid pressureapplied to PTO clutch 3 at a fast rate. Consequently, once PTO clutch 3is engaged, PTO 1, implement input and output shafts 5 and 13, and anyattached equipment immediately experience rapidly increasing torque andrapid acceleration along with related, undesirable stresses. (Incontrast, the unmodified PTO clutch control system would respond to thislarge differential by slowly increasing the hydraulic fluid pressureapplied to PTO clutch 3, thereby slowly increasing the torquetransmitted by the clutch, and PTO 1 would not experience the extremeacceleration or overshoot in the valve command.) This effect may beexacerbated if, as in some systems, the fast rate of increase commandfor the hydraulic fluid pressure is faster than the hydraulics responsetime. If this occurs, the system may determine that the hydraulic fluidpressure should be increased even faster, and so the resulting commandto increase the hydraulic fluid pressure may overshoot the optimum valueand produce excessive acceleration of PTO 1.

It should be noted that, while the above situation involvingover-running clutch 6 is the most common example of a circumstance inwhich spurious torque communicated through PTO clutch 3 may cause themodified PTO clutch control system to inappropriately increase hydraulicfluid pressure, this is not the only such circumstance. Any circumstancein which PTO 1 may begin rotation in response to spurious torquecommunicated through PTO clutch 3 when still not engaged may result inthe same inappropriate responses.

Accordingly, it would be advantageous to develop an improvement forexisting PTO clutch control systems that would enable the systems tocontrol the engagement of a PTO so that the PTO would accelerate toattain the speed (or a multiple or proportion of the speed, dependingupon gear reduction) of an engine both within and in not substantiallyless than a predetermined amount of time despite engine droop due toengagement of the PTO. It would also be advantageous to develop animprovement to the modified PTO clutch control system (as such system isdescribed above) whereby the control system would avoid producingexcessive acceleration of a PTO during PTO engagement even though (a) aPTO clutch acted in a non-ideal fashion to transmit torque before theclutch was engaged, (b) the PTO rotated in response to the transmittedtorque until the locking pins of an over-running clutch engaged thelocking notches of the over-running clutch, (c) the PTO clutch controlsystem sensed the PTO rotation, and (d) the PTO clutch control systemconsequently began to compare desired and actual accelerations of thePTO and adjust torque transmission by the PTO clutch in response to thedifferential between those accelerations. It would further beadvantageous if these improvements to the existing PTO clutch controlsystems could be implemented by making only minor changes to theexisting PTO clutch control systems.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus for controlling engagementof a power take-off shaft (PTO) on a vehicle with a power source, wherethe PTO is for supplying rotational motion from the vehicle to a pieceof equipment other than the vehicle. The apparatus includes a clutchwith an input shaft coupled to the power source and an output shaftcoupled to the PTO. The clutch includes a plurality of plates operableto translate through a distance during engagement of the clutch and,upon engagement of the clutch, controllably transmits torque between theinput and output shafts. The apparatus further includes an input speedsensor for sensing a quantity related to the angular velocity of theinput shaft and providing an input speed signal representative thereof,and an output shaft speed sensor for sensing angular velocity of theoutput shaft and providing an output shaft speed signal representativethereof. The apparatus also includes an electromechanical clutchcontroller coupled to the clutch for adjusting pressure of a hydraulicfluid applied to the clutch in response to control signals duringengagement of the PTO, and torque transmitted between the input andoutput shafts is responsive to the control signals. The apparatusfurther includes a control circuit coupled to the input and output shaftspeed sensors and the clutch controller. The control circuit receivesthe input and output shaft speed signals and provides the controlsignals in response thereto. During engagement of the PTO, the controlcircuit continuously determines an actual acceleration of the PTO basedupon the sensed angular velocity of the output shaft. Based upon adesired acceleration and the actual acceleration, the control circuitgenerates the control signals.

The control circuit continuously determines the desired acceleration ofthe PTO based upon the input speed signal. The control circuit alsoprovides control signals to cause the torque transmitted through theclutch to increase at a rate depending upon the difference between thedesired and actual accelerations when the actual acceleration is morethan a threshold proportion of the desired acceleration, and to causethe torque to increase at a rate independent of the difference when theactual acceleration is less than the threshold proportion of the desiredacceleration.

The present invention further relates to a method of controllingengagement of a power take-off shaft (PTO) on a vehicle having a powersource, the PTO supplying rotational motion to a piece of equipmentother than the vehicle. The method includes sensing a quantity relatedto the angular velocity of an input shaft of a clutch at an input speedsensor, sensing angular velocity of an output shaft of the clutch at anoutput shaft speed sensor, providing an input speed signalrepresentative of the sensed quantity related to the angular velocity ofthe input shaft and providing an output shaft speed signalrepresentative of the sensed angular velocity of the output shaft. Themethod also includes receiving the input and output shaft speed signalsat a control circuit, continuously determining an actual acceleration ofthe PTO based upon the output shaft speed signal and generating controlsignals using a desired acceleration and the actual acceleration. Themethod further includes adjusting operation of the clutch in response tothe control signals during engagement of the PTO, with torque beingcontrollably transmitted between the input and output shafts when theclutch is engaged.

The method further includes continuously determining the desiredacceleration of the PTO based upon the input speed signal. The controlsignals generated based upon the desired and actual accelerations may becontrol signals that, when the clutch is engaged, cause the torquetransmitted through the clutch to increase at a rate depending upon thedifference between the desired and actual accelerations when the actualacceleration is more than a threshold proportion of the desiredacceleration, and cause the torque to increase at a rate independent ofthe difference when the actual acceleration is less than the thresholdproportion of the desired acceleration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an exemplary arrangement fortransmitting power from an engine of an agricultural vehicle to a PTO ofthe agricultural vehicle;

FIG. 2 is a rear elevation view of a conventional over-running clutch;

FIG. 3A is a graphical representation of the actual and desired speedsof a PTO and engine speed of an agricultural vehicle during engagementof the PTO;

FIG. 3B is a graphical representation of the actual speed of a PTO of anagricultural vehicle during engagement of the PTO both before and aftera clutch for engaging the PTO is engaged;

FIG. 4 is a schematic block diagram of a PTO clutch control system on anagricultural vehicle;

FIG. 5 is a schematic block diagram representative of the circuitconfiguration for the controller of the control system shown in FIG. 4;

FIGS. 6A and 6B are flow charts representative of the control functionof the control system of FIG. 4;

FIG. 7 is a graphical representation of the actual and desired speeds ofa PTO and engine speed of an agricultural vehicle during engagement ofthe PTO; and

FIG. 8 is a section of a flow chart representative of one embodiment ofthe functionality of step 98 of the flow chart of FIG. 6B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 4., a PTO clutch control system 10 for an agriculturalvehicle 12 such as a tractor is shown. With the exception of PTO clutchcontrol system 10, tractor 12 may be a conventional agricultural tractorof the type including an engine 14 having conventional accessories suchas an alternator 16. Engine 14 is the power source for tractor 12 and,in addition to providing power to the drive wheels (not shown) oftractor 12, provides the power to apply rotational motion to amulti-plate, hydraulically-actuated PTO clutch 18. Depending uponwhether PTO clutch 18 is engaged, power from engine 14 may in turn betransmitted to an output shaft 32. Output shaft 32 is shown directlycoupled to a 1000 RPMPTO (high speed PTO) 33 and also is shown coupledto a 540 RPM PTO (low speed PTO) 35 by a reduction gear 37. Inalternative embodiments, high speed PTO 33 may be of another speedrating such as 750 RPM. While in alternate embodiments, high and lowspeed PTOs 33,35 may be provided at separate output terminals on tractor12, preferably each PTO may be employed at a single output terminal (onePTO may be substituted for the other).

Control system 10 includes a controller 20 (including, e.g., a digitalmicroprocessor such as the Intel TN83C51FA), a PTO on/off switch 22, anoutput shaft transducer 26 for providing a signal representing the speedof output shaft 32 to the controller, a PTO status switch 27, and anormally-closed, solenoid-operated, hydraulic, proportional clutchcontrol valve 28. Control system 10 also is coupled to alternator 16 andreceives a signal therefrom representing the speed of engine 14. Theengine speed is equal to or, depending upon gear reduction, a multipleor proportion of the speed of an input shaft 19 to PTO clutch 18 thatreceives power from engine 14 and transmits power to the clutch. Inalternate embodiments, a signal representative of the speed of inputshaft 19 (that is directly representative of the speed of engine 14) maybe obtained by way of an input shaft transducer 24 coupled to shaft 19instead of alternator 16. By way of example, transducers 24 and 26 maybe variable reluctance sensors. While status switch 27 is used toprovide information as to whether high speed PTO 33 or low speed PTO 35is being employed, in alternate embodiments a speed sensor (not shown)may instead be employed to provide such information. (In embodimentswhere tractor 12 has two separate output terminals for the two types ofPTOs, a status switch or speed sensor is not required.)

In addition to controlling PTO clutch 18, control system 10 may controla hydraulic brake 30 which inhibits rotational motion of output shaft 32when PTO clutch 18 is not fully engaged. Although brake 30 inhibitsrotational motion of output shaft 32 when PTO clutch 18 is not fullyengaged, the output shaft may nevertheless sometimes rotate in responseto torque transmitted by the clutch even when not fully engaged. Controlsystem 10 includes a hydraulic valve 34 connected to brake 30 by ahydraulic conduit 39. Valve 34 engages and disengages brake 30. Brake 30is biased to inhibit rotation of output shaft 32. Accordingly, valve 34is normally closed, and opened when brake 30 is to be released.Depending upon the application and the configuration of valve 28 and ahydraulic conduit 36 which connects valve 28 to PTO clutch 18, valve 34may be eliminated by connecting brake 30 directly to conduit 36.Accordingly, as valve 28 applied pressurized hydraulic fluid to engagePTO clutch 18, the pressurized fluid would also release brake 30. Byconfiguring conduits 36 and 39 appropriately, the engagement of PTOclutch 18 and releasing of brake 30 can be synchronized to avoidengaging PTO clutch 18 without appropriately releasing brake 30.

Alternator 16 and transducer 26 are coupled to digital inputs ofcontroller 20 by, respectively, electrical conductors 21,29 andconditioning circuits 79,38, which may be integral to controller 20. (Inalternative embodiments in which signals regarding input shaft 19 areprovided by transducer 24, an electrical conductor 25 along withconditioning circuit 38 may be employed.) Conditioning circuits 79,38filter radio and other undesirable frequencies of interference from thesignals produced by alternator 16 and transducer 26 (or, in alternateembodiments, transducer 24) and introduced in conductors 21,29 (or, inalternate embodiments, conductor 25). Additionally, conditioningcircuits 79,38 place the signals produced by alternator 16 andtransducer 26 (or transducer 24) within a 5 V range and provide thesesignals with a generally squarewave configuration which can beappropriately sampled by controller 20. Accordingly, the signals appliedto controller 20 by alternator 16 (or transducer 24) and transducer 26have a generally squarewave configuration with a frequency proportionalto the rotational speed of input shaft 19 (or of engine 14) and outputshaft 32, respectively.

Switches 22 and 27 each include an associated conditioning circuit 40and 42, respectively, which may be integral to controller 20. Dependingupon the application, circuits 40 and 42 may provide signal inversionand appropriate filtering to eliminate switch bounce. However, dependingupon the type of controller 20 used, circuits 40 and 42 may beeliminated. The signals produced by switches 22 and 27 are applied todigital inputs of controller 20 via electrical conductors 23 and 31,respectively.

Hydraulic valves 28 and 34 are coupled to digital outputs of controller20 by appropriate amplification and signal conditioning circuits 44 and46, which may be integral to controller 20, and electrical conductors 48and 50, respectively. As will be discussed in detail below, controller20 applies an analog current signal to valve 28 via electrical conductor48 and circuit 44, and applies a digital on/off signal to valve 34 viaelectrical conductor 50 and circuit 46. Due to the nature of thesolenoids which operate valves 28 and 34, amplification and isolationcircuits 44 and 46 are required to produce control signals havingsufficient current and voltage to operate valves 28 and 34.Additionally, due to inductive kickbacks which may potentially beproduced by the solenoids of valves 28 and 34, isolation may be requiredin circuits 44 and 46 to protect controller 20. While controller 20 isconfigured to apply an analog current signal to valve 28, in alternativeembodiments an analog voltage signal, a pulse-width modulated (PWM)current signal or a PWM voltage signal may be provided instead to valve28. In each case, the magnitude of the signal provided (which, in thecase of a PWM current or voltage signal, is the time-average magnitudeof the signal and therefore depends upon the duty cycle or pulse widthof the signal) is proportional to the desired pressure from valve 28.

Turning to the operation of valve 28, valve 28 is a proportionalhydraulic valve which applies hydraulic fluid to PTO clutch 18 from asystem hydraulic fluid source 52 at a pressure which is related to(e.g., proportional to) the current applied to the solenoid associatedwith valve 28. Thus, the pressure of the fluid applied to PTO clutch 18via hydraulic conduit 36 by valve 28 is controlled by applying avariable current signal to valve 28. In alternate embodiments, thepressure may be controlled by applying a variable voltage signal, a PWMcurrent signal, or PWM voltage signal to valve 28. Where a PWM signal isapplied to the solenoid of valve 28 to control the pressure of thehydraulic fluid applied to PTO clutch 18, the pressure of the fluid isproportional to the pulse width of the PWM signal produced by controller20.

As discussed above, PTO clutch 18 is a multi-plate hydraulic clutch.This type of clutch is capable of transferring a torque from input shaft19 to output shaft 32, where the torque is generally proportional to thepressure of the hydraulic fluid applied to PTO clutch 18. Output shaft32 is shown directly coupled to 1000 RPM PTO (high speed PTO) 33 andalso is shown coupled to 540 RPM PTO (low speed PTO) 35 by reductiongear 37. In alternative embodiments, high speed PTO 33 may be of anotherspeed rating such as 750 RPM. Accordingly, the torque transferredbetween shafts 19 and 32 will be generally proportional to the magnitudeof the analog current signal applied from controller 20 to the solenoidof valve 28. (In alternate embodiments where an analog voltage signal, aPWM current signal, or a PWM voltage signal is provided to valve 28, thetorque transferred between shafts 19 and 32 also will be generallyproportional to the magnitude of the applied signal, which in the caseof a PWM signal is proportional to the duty cycle or pulse width of thesignal.) Ideally, it may be convenient to have the torque transferredbetween shafts 19 and 32 exactly proportional to the magnitude of thecurrent signal applied to valve 28; however, in mechanical systems, sucha relationship may be difficult to obtain. Accordingly, controller 20 isprogrammed to compensate for the inability to obtain suchproportionality, and overall non-linearity in the electronics andmechanism of the control system 10.

Also shown in FIG. 4 is an implement 17 that may be attached to(typically, towed by) tractor 12. Implement 17 includes equipment (notshown) that is operated by way of power from tractor 12. The equipmentmay perform one or more actions upon a field, such as planting ortilling. Implement 17 is capable of receiving power from tractor 12 viaan implement input shaft 51 coupled to high speed PTO 33 via a coupler47. When PTO clutch 18 is engaged and is transmitting power from engine14 to output shaft 32 and high speed PTO 33, power is also thentransmitted to implement input shaft 51. In addition to implement inputshaft 51, implement 17 also include an implement output shaft 85 thatcouples, and transmits power from, the implement input shaft to theequipment. Implement input shaft 51 and implement output shaft 85 arecoupled via an over-running clutch 87. Over-running clutch 87, asdescribed above, allows implement output shaft 85 to continue to rotatefreely even when implement input shaft 51 is not rotating, and allowsthe implement output shaft to rotate at a higher angular velocity thanthe implement input shaft. As described above, if locking pins andnotches (not shown) of over-running clutch 87 are not engaged, implementinput shaft 51 must rotate a portion of a rotation to engage the pinswith the notches before the over-running clutch will transmit power fromthe input shaft to implement output shaft 85. Implement input shaft 51is coupled to high speed PTO 33. In alternate embodiments, a similarimplement input shaft may be coupled to low speed PTO 35 by way of asecond coupler (not shown).

Referring to FIG. 5, controller 20 includes a memory circuit 54 havingRAM and ROM, and is configured (programmed) to provide the operations ofa speed sensing circuit 56, a frequency interface circuit 57, a timingcircuit 58, a switch status monitoring circuit 60, a signal processingcircuit 62, and a valve control signal output circuit 64. The directionand channels for data flow between circuits 54, 56, 57, 58, 60, 62 and64 are shown in FIG. 5. The ROM of memory circuit 54 stores those valuesrequired for initialization of control system 10, and the constantsrequired for the operation of certain programs run by controller 20. TheRAM of memory circuit 54 provides the temporary digital storage requiredfor controller 20 to execute the control system program.

Frequency interface circuit 57 and speed sensing circuit 56 receivesignals from alternator 16 and transducer 26 that are applied toconductors 21 and 29, and convert the signals to digital valuesrepresentative of the rotational speeds of engine 14 (or input shaft 19)and output shaft 32, respectively. (In alternative embodiments, speedsensing circuit 56 may receive signals from transducer 24 that areapplied to conductor 25, and convert those signals to digital valuesrepresentative of the rotational speed of input shaft 19, in place of orin addition to frequency interface circuit 57, alternator 16 andconductor 21.) Insofar as the output of alternator 16 is a square-wave,frequency interface circuit 57 may operate as a timing interface thatmeasures the time between pairs of edges of the square wave. Timingcircuit 58 includes counters which are utilized by signal processingcircuit 62 while executing the programming represented by the flowcharts of FIGS. 6A and 6B. Switch status monitoring circuit 60 convertsthe signals applied by switches 22 and 27 to conductors 23 and 31 todigital values representative of the status of these switches. Valvecontrol signal output circuit 64 produces an analog current signalapplied to the solenoid of valve 28 via conductor 48 and circuit 44having an appropriate magnitude, and produces the on/off signal appliedto valve 34 via conductor 50 and circuit 46. As briefly discussed below,the program executed by controller 20 in the present embodiment isexecuted at 100 Hz (although, in alternate embodiments the program couldbe executed at other frequencies). (In an alternate embodiment in whichvalve 28 is provided with a PWM current or voltage signal, valve controlsignal output circuit 64 would produce a 400 Hz PWM current or voltagesignal having an appropriate pulse width. Assuming the same programexecution frequency of 100 Hz, the pulse width of the signal fromcircuit 64 would be updated every 10 milliseconds or every 4 cycles ofthe PWM signal.)

The operation of signal processing circuit 62 will now be described indetail in reference to FIGS. 6A, 6B, 7 and 8. FIGS. 6A and 6B representthe operational steps of the program run by controller 20. Upon startup(step 66), controller 20 reads the ROM of memory circuit 54 andinitializes the counter in timing circuit 58 to a number of countsrepresentative of 6 seconds. In addition, controller 20 initializesthose other variables and constants which may be utilized in theprogramming of controller 20 (step 68). In step 70, circuit 62 reads thedigital value representative of the status of PTO on/off switch 22 fromcircuit 60, and returns if the switch has not been closed. If switch 22is closed, after it was detected open, circuit 62 executes the stepsrequired to begin engagement of output shaft 32 (and high speed and lowspeed PTOs 33 and 35).

In step 71, circuit 62 accesses circuit 60 to determine if switch 22 wasopened and closed. If switch 22 was opened and closed, circuit 62 setsthe counts in timing circuit 58 to a number representative ofapproximately 2 seconds (step 73) and then advances to step 72. Ifswitch 22 was not opened and closed, circuit 62 advances immediately tostep 72.

In step 72, circuit 62 reads the digital value representative of thestatus of switch 27 from circuit 60 and determines whether or not thePTO is operating as a high speed PTO or a low speed PTO. If switch 27produces a signal representative of a low speed PTO, a LOW PTO flag isset. In step 74, circuit 62 determines whether the LOW PTO flag is setand, if so, circuit 62 calculates the torque limit for PTO clutch 18 atstep 75 and stores a value in the RAM of circuit 54 representative ofthe maximum magnitude of the analog current signal (or, in alternateembodiments, the maximum magnitude of the analog voltage signal ormaximum pulse width of the PWM current or voltage signal) to be appliedto the solenoid of valve 28 during operation of low speed PTO 35. Themaximum magnitude of the analog current signal depends upon theconfiguration of tractor 12, and is set so that the torque transferredby PTO clutch 18 is less than the maximum torque at which low speed PTO35 will fail.

Since the reduction required to reduce the speed of low speed (540 RPM )PTO 35 to approximately 50% of the speed of high speed (1000 RPM) PTO 33is approximately 2 to 1, a torque is applied to the low speed PTO thatis approximately twice as large as the torque that is applied to thehigh speed PTO given a specific engine torque. Accordingly, the maximumpressure applied to PTO clutch 18 through valve 28 during operation oflow speed PTO 35 to transmit a given torque to the low speed PTO isapproximately 50% of the maximum pressure applied to the clutch throughvalve 28 during the operation of high speed PTO 33 to transmit the sametorque to that PTO. This pressure is controlled by changing themagnitude of the analog current signal applied (or, in alternateembodiments, controlled by changing the magnitude of the analog voltagesignal applied or the pulse width of the PWM signal applied). Themaximum magnitude of the analog current signal (or, in alternateembodiments, the maximum magnitude of the analog voltage signal or themaximum pulse width of the PWM signal applied to valve 28) associatedwith low speed PTO 35 is stored in the ROM of circuit 54. At step 74, ifcircuit 62 determines that the LOW PTO flag is not set, circuit 62 willutilize the maximum current value stored in circuit 54 which isassociated with the maximum torque PTO clutch 18 can transfer betweeninput shaft 19 and low speed PTO 35 during operation of the low speedPTO, without causing failure of the low speed PTO due to torqueoverload.

In step 76, circuit 62 reads the digital values representative of therotational speeds of input shaft 19 (or engine 14) and output shaft 32from circuits 57 and 56. In step 78, circuit 62 compares the speedsinput shaft 19 (or engine 14) and output shaft 32. If the shaft speedsare the same, circuit 62 resets timing circuit 58 to a countrepresentative of 2 seconds, and sets a STEADY STATE flag (step 80).Subsequently, circuit 62 loops to execute step 102 and the stepsbeginning at step 100. At step 102 the current magnitude value isincreased by 1.00%. If the shaft speeds are different, processingcontinues at step 82.

In step 82, circuit 62 determines whether or not the STEADY STATE flagis set. If the STEADY STATE flag is set, circuit 62 determines if thespeed difference between shafts 19 and 32 is greater than five percent(5%) (step 83). If the speed difference is greater than five percent,the time counter is decremented by 10 milliseconds (step 84), andcircuit 62 jumps to the programming associated with steps 102 and then100. If the speed difference is not greater than five percent, circuit62 jumps immediately to the programming associated with steps 102 andthen 100. If the STEADY STATE flag is not set, circuit 62 goes to step86 wherein circuit 62 decrements the counter of circuit 58 by countsrepresentative of 10 milliseconds. (The programming represented by theflow charts of FIGS. 6A and 6B runs at a rate of approximately 100 Hz.Accordingly, to decrement the timer counter in circuit 58, the countermust be decremented by the number of counts associated with 10milliseconds.)

In step 88, circuit 62 reads the value representative of the rotationalspeed of output shaft 32 to determine whether or not shaft 32 is moving.If shaft 32 is moving, circuit 62 applies a digital signal to circuit64, where circuit 64 responds to the signal by applying a signal toconductor 50 which causes valve 34 to release brake 30 (step 92). Atstep 90, if shaft 32 is not moving, circuit 62 reads the time from timercircuit 58 associated with the times since PTO on/off switch 22 wasclosed and sets the current magnitude value to a predeterminedpercentage (e.g. 20%) of the maximum current magnitude value either setat step 75 in the case of operation of low speed PTO 35 (at 540 RPM ),or read from circuit 54 in the case of operation of high speed PTO 33(at 1000 RPM), if switch 22 has been closed for 300 milliseconds orless. If the time is greater than 300 milliseconds, the currentmagnitude value is increased by 0.1% for each 10 millisecond incrementof time elapsed subsequent to switch 22 being closed for 300milliseconds. After setting the current magnitude value at step 90,circuit 62 jumps to step 104.

In general, steps 88 and 90 are provied produce smooth engagement of PTOclutch 18. More specifically, before the plates of PTO clutch 18 engage,a certain volume of hydraulic fluid must be provided to PTO clutch 18before the clutch plates of PTO clutch 18 travel through the distancerequired to engage the clutch plates. During this clutch fillingprocess, it is undesirable to apply hydraulic fluid to the clutch at afixed or undesirably high pressure since the clutch will abruptly applytorque from shaft 19 to shaft 32. Such an abrupt application of torquecan potentially cause damage to output shaft 32 or an associatedimplement connected to the output shaft. By initiating the filling ofPTO clutch 18 with a pressure equivalent to the pre-stress force appliedby the clutch springs, the clutch plates move relatively slowly towardengagement, and the pressure is increased gradually until engagement.This process prevents the abrupt transfer of torque from shaft 19 toshaft 32.

In step 94, circuit 62 calculates a desired acceleration by dividing thespeed at shaft 19 by 2 seconds. In general, step 94 is the start of theprocess for controlling PTO clutch 18 to accelerate output shaft 32relative to shaft 19 until the speed of shaft 32 reaches its steadystate speed (no slip with respect to PTO clutch 18) which equals or isproportional to the speed of shaft 19. The desired acceleration of shaft32 in step 94 is calculated based upon 2 seconds, which was selectedbased upon experimentation to provide optimum acceleration of shaft 32.However, depending upon the system configuration, this time period maybe varied according to the particular tractor and PTO application. Thecalculated acceleration serves as a reference for accelerating shaft 32relative to shaft 19 in step 96.

As described above, the prior art PTO clutch control system onlycalculates the desired acceleration once, just as the PTO engagementprocess is beginning. In contrast, in the present embodiment of theinvention, the PTO clutch control system repeatedly calculates thedesired acceleration. As shown in FIGS. 6A and 6B, the control systemprogram repeatedly cycles through step 94 once PTO on/off switch 22 hasbeen closed (in step 70) and so long as the speeds of input shaft 19 andoutput shaft 32 are different (as determined in step 78), the STEADYSTATE flag is not set (as determined in step 82), and the output shaftis moving (as determined in step 88). In the present embodiment (unlikethe prior art), the desired acceleration is recalculated each time thePTO clutch control system cycles through step 94 (instead of only thefirst time). The desired acceleration is repeatedly calculated bydividing the current speed of shaft 19, or another quantity related toengine speed, by the desired time of engagement, which is 2 seconds inthe present embodiment. Although in alternate embodiments the frequencyof recalculation may vary (or the recalculation may occur at a frequencyless than the frequency at which the control system program cyclesthrough step 94), in the present embodiment the desired acceleration isrecalculated at the same frequency as the control system program cyclesthrough step 94, which (as stated above) is approximately 10milliseconds. Therefore, the recalculation occurs with sufficientrapidity that the desired acceleration is effectively continuouslyrecalculated to reflect changes in the speed of input shaft 19 (that is,changes in engine speed).

Referring to FIG. 7, examples of the desired and actual speeds foroutput shaft 32 (i.e., PTO speed), and engine speed (i.e., the speed ofinput shaft 19), as measured or determined by the PTO clutch controlsystem of the present embodiment, are plotted against time. Four desiredspeed curves are shown. The four speed curves are determined based uponthe engine speed (or speed of input shaft 19) as measured at four times,t₁, t₂, t₃ and t₄ and are labeled as, and referred to below as,respectively, the “desired PTO speed #1”, “desired PTO speed #2”,“desired PTO speed #3” and “desired PTO speed #4” curves. Forconvenience, only four desired speed curves are shown in FIG. 7. Asdiscussed above, the desired accelerations in the present embodiment areactually recalculated approximately every 10 milliseconds (effectivelycontinuously), and so FIG. 7 is meant to be a symbolic description ofthe actual operation of the PTO clutch control system, in which thereare many more than four desired speed curves. Also, it is for generalitythat the four desired speed curves are shown as being calculated at fourtimes (times t₁-t₄ that are not equidistant from one another. Althoughalternative embodiments may vary, in the present embodiment, the desiredaccelerations (in contrast to FIG. 7) are recalculated at a constantfrequency as the PTO clutch control system repeatedly cycles throughstep 94.

As shown in FIG. 7, output shaft 32 begins to rotate at time t₁, and thespeed of the output shaft equals the speed of input shaft 19 (or theengine speed) at time t₅ (lock-up). Also as shown, the speed of inputshaft 19 (and that of the engine) does not remain constant as powerbegins to be transferred to output shaft 32 but instead decreases ordroops. Consequently if the actual speed of output shaft 32 were toincrease in accordance with the desired PTO speed #1 curve, which isdetermined based upon the initial engine speed at time t₁, the shaftwould attain the speed of input shaft 19 (i.e., the engine speed) in atime significantly shorter than the desired time of engagement (the timeinterval between times t₅ and t₁, i.e., 2 seconds). Instead of attainingthe speed of input shaft 19 at time t₅, the shaft would attain the speedof the input shaft at the time at which, as shown in FIG. 7, the desiredPTO speed #1 curve crosses the engine speed curve.

The present embodiment of the invention avoids this excessive engagementrate by adjusting the desired speed curve as engine speed decreases. Asshown in FIG. 7, at times t₂, t₃ and t4, the desired acceleration isrecalculated (i.e., at step 94 of the control system program) and thedesired speed curve changes, respectively, to the desired PTO speed #2,desired PTO speed #3 and desired PTO speed #4 curves. As describedbelow, in the present embodiment the actual acceleration of output shaft32 is adjusted as the desired speed curve changes (more specifically,the actual acceleration is adjusted based upon the difference betweenthe actual and desired accelerations). Insofar as the actualacceleration of output shaft 32 is adjusted to reflect the new desiredspeed curves, the output shaft speed increases at a rate such that itwill approach the speed of input shaft 19 (i.e., the engine speed) atapproximately t₅ (i.e., within the desired time of engagement, 2seconds), as shown in FIG. 7, and not substantially before t₅.

Turning to step 96, the actual acceleration of output shaft 32 iscalculated by reading the current speed of the output shaft from circuit56, and the speed of the output shaft monitored during the previous loopthrough steps 70-108. Steps 70-108 are executed every 10 milliseconds;thus, the output shaft acceleration is the change in output shaft speedbetween program loops divided by 10 milliseconds. Once the actualacceleration of output shaft 32 is calculated in step 96, it may becompared with the desired acceleration calculated in step 94. If theactual acceleration of output shaft 32 is greater than or equal to thedesired acceleration, the current magnitude value is not changed (theprogram proceeds from step 96 to step 100). In certain systems, it maybe desirable to reduce the current magnitude value when the actualacceleration of output shaft 32 is greater than the desiredacceleration. However, this type of control may cause hunting, and thus,an acceleration of output shaft 32 which is not smooth. Accordingly, inthe presently preferred embodiment of control system 10, the currentmagnitude value is not modified when the actual acceleration of outputshaft 32 exceeds the desired acceleration.

However, if the actual acceleration of output shaft 32 is less than thedesired shaft acceleration, the present current magnitude is increased,in step 98. The manner in which current magnitude changes may vary fordifferent control systems. As stated above, a first control system wouldincrease, at step 98, the current magnitude by 0.1% if the desiredacceleration exceeds the actual acceleration (the “unmodified PTO clutchcontrol system”). A second control system would employ a proportional(more accurately, pseudo-proportional) adjustment algorithm (the“modified PTO clutch control system”) in which: if the actualacceleration of the PTO is less than the desired acceleration butgreater than two-thirds of the desired acceleration, the control systemincreases the torque transmitted by the PTO clutch at a slow rate; ifthe actual acceleration is less than two-thirds of the desiredacceleration but greater than one-third of the desired acceleration, thecontrol system increases the torque transmitted at a medium rate; and ifthe actual acceleration is less than one-third of the desiredacceleration, the control system increases the torque transmitted at afast rate. However, while either of these control systems (or theprogramming contained therein) may be employed in conjunction with thecontrol system described above in which the desired accelerations arerepeatedly recalculated (i.e., such that the desired speed curve changeswith engine speed), in the present embodiment of the invention a thirdsystem is employed.

Referring to FIG. 8, step 98 (of the program shown in FIGS. 6A and 6B)is shown to constitute a modified proportional adjustment algorithm. Instep 98 a, the program determines whether the actual acceleration isbetween the desired acceleration and two-thirds of the desiredacceleration. If so, the program increases the current magnitude at aslow rate in step 98 d. If not, the program proceeds to step 98 b, inwhich the program determines whether the actual acceleration is betweentwo-thirds of the desired acceleration and one-third of the desiredacceleration. If so, the program increases the current magnitude at amedium rate in step 98 e. If not, the program proceeds to step 98 c, inwhich the program determines whether the actual acceleration is betweenone-third of the desired acceleration and one-sixth of the desiredacceleration. If so, the program increases the current magnitude at afast rate in step 98 f. If not, the actual acceleration is between zeroand one-sixth of the desired acceleration, and the program proceeds tostep 98 g in which the current magnitude is increased at a slow rate.(It should be noted that the program may be designed to treat actualaccelerations that exactly equal two-thirds, one-third or one-sixth ofthe desired acceleration as if the actual accelerations were above orbelow these levels.)

A significant characteristic of this modified proportional adjustmentalgorithm is that the modified proportional adjustment algorithm (a)determines whether the actual acceleration is below a minimum thresholdproportion of the desired acceleration, and (b) increases the currentmagnitude at a slow rate if the actual acceleration is below the minimumthreshold proportion even though the actual acceleration issignificantly less than the desired acceleration. That is, in thepresent embodiment, the modified proportional adjustment algorithmdetermines in step 98 c whether the actual acceleration is belowone-sixth of the desired acceleration and, if so, increases the currentmagnitude at a slow rate in step 98 g.

This feature of the modified proportional adjustment algorithmalleviates the problems described above that are associated with thepossible spurious rotation of output shaft 32 due to premature deliveryof torque by PTO clutch 18 (before the clutch is fully engaged) that mayoccur, for example, before over-running clutch 87 is locked. This isbecause typically, once PTO clutch 18 is engaged and output shaft 32 isbeing accelerated, the output shaft would not have an actualacceleration less than one-sixth of the desired acceleration. Further,typically, PTO clutch 18 is not capable of delivering sufficient torquewhen the clutch is not fully engaged as to cause output shaft 32 toaccelerate at a rate greater than one-sixth of any of the desiredaccelerations that may be calculated by control system 10. Therefore,the modified proportional adjustment algorithm fulfills the two goals of(a) causing the current magnitude to increase at a fast rate when theactual acceleration of output shaft 32 is significantly less than thedesired acceleration and yet (b) not causing the current magnitude toincrease at a fast rate when PTO clutch 18 is still not fully engaged.

While in the present embodiment, the ratios of actual acceleration todesired acceleration that determine the current magnitude increase ratesare set at two-thirds, one-third and one-sixth, in alternate embodimentsthese ratios may be set at different levels. Indeed, different PTOclutch control systems may have a variety of different proportionaladjustment algorithms that distinguish among more (or less) than fourranges (of ratios of actual acceleration to desired acceleration) and inwhich the control systems provide finer (or less fine) gradations ofincreases in the current magnitude. (Fully proportional control may alsobe appropriate in certain embodiments.) Also, the exact values for the“slow”, “medium” and “fast” rates of current increase may vary dependingupon the embodiment, although the “fast” rate of increase will typicallybe the fastest rate at which the mechanical clutch can predictablyincrease torque in response to commands from the control system toincrease pressure. It should be noted that, while the present embodimentof the invention combines both the functionality of this modifiedproportional adjustment algorithm and the above-described repeated(continuous) recalculation of the desired acceleration (and modificationof the desired speed curve), the modified proportional adjustmentalgorithm of step 98 may be employed even when the desired accelerationis only calculated once.

At time t₅, the speeds of input shaft 19 (or engine 14) and output shaft32 are equal or proportional, and circuit 62 executes steps 100, 101 and102 to ramp up the current signal to produce a clutch pressure in PTOclutch 18 associated with the maximum torque to be transmitted betweeninput and output shaft 19, 32. In step 100, the current signal iscompared with the maximum current value set and determined at step 75 incase of operation at low speed (540 RPM ), and the maximum current valuestored in circuit 54 in case of operation at high speed (1000 RPM). Ifthe present current value set at step 98 or step 102 is greater than themaximum current value, the current value is set to the maximum currentvalue (step 101).

In step 104, circuit 62 checks the count of the timer in circuit 58 to30 determine whether or not the timer has timed out. If the timer equals0, then either motion of output shaft 32 did not occur within 6 seconds(timer count at initialization), or the speed difference between shaftsinput and output shafts 19, 32 subsequent to time t₅ (lock-up) has beengreater than 5% for more than 2 seconds which indicates undesirableslippage in PTO clutch 18. In step 104, circuit 62 also determines ifthe speed of input shaft 19 has gone below 500 RPM. If either the timercount has reached 0 or the speed of input shaft 19 has gone below 500RPM, circuit 62 sets the current magnitude to zero (step 105). In step106, circuit 62 applies the present current magnitude value to circuit64. In response, circuit 64 applies an analog current signal to valve 28via conductor 48 at a frequency of 400 Hz with a magnitude correspondingto the present current magnitude value which will be updated upon thenext execution of steps 70 through 106. In step 108, circuit 62 returnsto the execution of step 70.

In alternate embodiments in which a voltage or PWM current or voltagesignal is applied to valve 28, a similar routine would be employed inwhich the voltage magnitude or pulse width (of a PWM signal) isincreased or otherwise controlled in place of current magnitude, inorder to respond to the difference between the desired and actualaccelerations, to limit the maximum value of the 15 voltage magnitude orpulse width of the signal applied, and to perform the other operationsdescribed above.

Although only a few exemplary embodiments of this invention have beendescribed above, those skilled in the art will readily appreciate thatmany modifications are possible in the exemplary embodiments withoutmaterially departing from the novel teachings and advantages of thisinvention. For example, developments in PTO clutches may make electricclutches cost effective for PTO applications. Accordingly, hydraulic PTOclutch 18 and control valve 28 may potentially be replaced with anassociated electric clutch and electric clutch control circuit. As isreadily indicated, the invention can be employed in a variety of workvehicles that employ PTOs that must be engaged and disengaged. Further,the number and types of control elements and sensor elements may varyinsofar as they continue to accomplish similar functions related to theengagement of PTOs. Accordingly, all such modifications are intended tobe included within the scope of the invention as defined in thefollowing claims. Other substitutions, modifications, changes andomissions may be made in the design, operating conditions andarrangement of preferred and alternative embodiments without departingfrom the spirit of the invention as expressed in the appended claims.

What is claimed is:
 1. In a vehicle having a power source for producingrotational motion, an apparatus for controlling engagement of a powertake-off shaft (PTO) for supplying rotational motion to a piece ofequipment other than the vehicle, the apparatus comprising: a clutchincluding an input shaft coupled to the power source and an output shaftcoupled to the PTO, the clutch including a plurality of plates operableto translate through a distance during engagement of the clutch and,upon engagement of the clutch, controllably transmitting torque betweenthe input and output shafts; an input speed sensor for sensing aquantity related to the angular velocity of the input shaft andproviding an input speed signal representative thereof; an output shaftspeed sensor for sensing angular velocity of the output shaft andproviding an output shaft speed signal representative thereof; anelectromechanical clutch controller coupled to the clutch for adjustingpressure of a hydraulic fluid applied to the clutch in response tocontrol signals during engagement of the PTO, wherein torque transmittedbetween the input and output shafts is responsive to the controlsignals; and a control circuit coupled to the input and the output shaftspeed sensors and the clutch controller, the control circuit configuredto receive the input and the output shaft speed signals and to providethe control signals in response thereto, and wherein, during engagementof the PTO, the control circuit continuously determines a desiredacceleration of the PTO based upon the input speed signal andcontinuously determines an actual acceleration of the PTO shaft basedupon the sensed angular velocity of the output shaft, and the controlcircuit generates the control signals using the desired and actualaccelerations, wherein the control circuit provides control signals tocause the torque transmitted through the clutch to increase at a ratedepending upon the difference between the desired and actualaccelerations when the actual acceleration is more than a thresholdproportion of the desired acceleration, and to cause the torque toincrease at a rate independent of the difference when actualacceleration is less than the threshold proportion of the desiredacceleration.
 2. The apparatus of claim 1, wherein the control circuitgenerates the control signals so that the angular velocity of the outputshaft increases to equal the angular velocity of the input shaft withina particular time period.
 3. The apparatus of claim 1, wherein thethreshold proportion at a given time is one-sixth of the desiredacceleration at that time, and wherein the control signal provided bythe control circuit when the actual acceleration is less than thethreshold proportion is a slow increase control signal.
 4. The apparatusof claim 3, wherein the control signals provided by the control circuitwhen the actual acceleration is more than the threshold proportion ofthe desired acceleration include a slow increase control signal, amedium increase control signal and a fast increase control signal,wherein the control circuit provides the slow increase control signal ifthe actual acceleration is less than the desired acceleration and morethan two-thirds of the desired acceleration, wherein the control circuitprovides the medium increase control signal if the actual accelerationis less than two-thirds of the desired acceleration and more thanone-third of the desired acceleration, and wherein the control circuitprovides the fast increase control signal if the actual acceleration isless than one-third of the desired acceleration and more than thethreshold proportion.
 5. In a vehicle having a power source forproducing rotational motion, an apparatus for controlling engagement ofa power take-off shaft (PTO) for supplying rotational motion to a pieceof equipment other than the vehicle, the apparatus comprising: a clutchincluding an input shaft coupled to the power source and an output shaftcoupled to the PTO, the clutch including a plurality of plates operableto translate through a distance during engagement of the clutch and,upon engagement of the clutch, controllably transmitting torque betweenthe input and output shafts; an input speed sensor for sensing aquantity related to the angular velocity of the input shaft andproviding an input speed signal representative thereof; an output shaftspeed sensor for sensing angular velocity of the output shaft andproviding an output shaft speed signal representative thereof; anelectromechanical clutch controller coupled to the clutch for adjustingpressure of a hydraulic fluid applied to the clutch in response tocontrol signals during engagement of the PTO, wherein torque transmittedbetween the input and output shafts is responsive to the controlsignals; and a control circuit coupled to the input and the output shaftspeed sensors and the clutch controller, the control circuit configuredto receive the input and the output shaft speed signals and to providethe control signals in response thereto, and wherein, during engagementof the PTO, the control circuit continuously determines an actualacceleration of the PTO based upon the sensed angular velocity of theoutput shaft, and the control circuit generates the control signalsusing a desired acceleration and the actual acceleration, wherein thecontrol circuit provides control signals to cause the torque transmittedthrough the clutch to increase at a rate depending upon the differencebetween the desired and actual accelerations when the actualacceleration is more than a threshold proportion of the desiredacceleration, and to cause the torque to increase at a rate independentof the difference when the actual acceleration is less than thethreshold proportion of the desired acceleration.
 6. The apparatus ofclaim 5, wherein the threshold proportion at a given time is one-sixthof the desired acceleration at that time, and wherein the control signalprovided by the control circuit when the actual acceleration is lessthan the threshold proportion is a slow increase control signal.
 7. Theapparatus of claim 6, wherein the control signals provided by thecontrol circuit when the actual acceleration is more than the thresholdproportion of the desired acceleration include a slow increase controlsignal, a medium increase control signal and a fast increase controlsignal, wherein the control circuit provides the slow increase controlsignal if the actual acceleration is less than the desired accelerationand more than two-thirds of the desired acceleration, wherein thecontrol circuit provides the medium increase control signal if theactual acceleration is less than two-thirds of the desired accelerationand more than one-third of the desired acceleration, and wherein thecontrol circuit provides the fast increase control signal if the actualacceleration is less than one-third of the desired acceleration and morethan the threshold proportion.
 8. In a vehicle having a power source forproducing rotational motion, an apparatus for controlling engagement ofa power take-off shaft (PTO) for supplying rotational motion to a pieceof equipment other than the vehicle, the apparatus comprising: a clutchincluding an input shaft coupled to the power source and an output shaftcoupled to the PTO, the clutch including a plurality of plates operableto translate through a distance during engagement of the clutch and,upon engagement of the clutch, controllably transmitting torque betweenthe input and output shafts; an input speed sensing means for sensing aquantity related to the angular velocity of the input shaft andproviding an input speed signal representative thereof; an output speedsensing means for sensing a quantity related to the angular velocity ofthe output shaft and providing an output speed signal representativethereof; a clutch control means coupled to the clutch for adjustingoperation of the clutch in response to control signals during engagementof the PTO, wherein torque transmitted between the input and outputshafts is responsive to the control signals; and a speed control meanscoupled to the input and the output speed sensing means and the clutchcontrol means, the speed control means for receiving the input and theoutput speed signals, for continuously determining an actualacceleration of the PTO based upon the output speed signal forgenerating the control signals using a desired acceleration based uponthe input speed sign and the actual acceleration, and for providing thecontrol signals to the clutch control means in response thereto, whereinthe speed control means provides the control signals to cause the torquetransmitted through the clutch to change at a fast rate when thedifference between the desired and actual accelerations is large exceptwhen the difference between the desired and actual accelerationsindicates that the clutch is not yet engaged.
 9. In a vehicle having apower source for producing rotational motion, a method of controllingengagement of a power take-off shaft (PTO) for supplying rotationalmotion to a piece of equipment other than the vehicle, the methodcomprising the steps of: sensing a quantity related to the angularvelocity of an input shaft of a clutch at an input speed sensor; sensingangular velocity of an output shaft of the clutch at an output shaftspeed sensor; providing an input speed signal representative of thesensed quantity related to the angular velocity of the input shaft;providing an output shaft speed signal representative of the sensedangular velocity of the output shaft; receiving the input and the outputshaft speed signals at a control circuit; determining a desiredacceleration of the PTO based upon the input speed signal; continuouslydetermining an actual acceleration of the PTO based upon the outputshaft speed signal; generating control signals based upon the desiredand actual accelerations that, when the clutch is engaged, cause thetorque transmitted through the clutch to increase at a rate dependingupon the difference between the desired and actual accelerations whenthe actual acceleration is more than a threshold proportion of thedesired acceleration, and cause the torque to increase at a rateindependent of the difference when the actual acceleration is less thanthe threshold proportion of the desired acceleration; and adjustingoperation of the clutch in response to the control signals duringengagement of the PTO, wherein torque is controllably transmittedbetween the input and output shafts when the clutch is engaged.